With single channel rate of wavelength division multiplexing optical transmission system increasing continually (e.g. the signal rates of main transmission networks are mostly updated from 2.5 Gb/s to 10 Gb/s at present), as one of the important physical characteristics of single mode fiber of optical transmission media, dispersion has become a main factor restricting transmission distance of high speed optical transmission system.
Taken the dispersion characteristic of common G.652 single mode fiber used in wavelength division multiplexing system (WDMS) as an example, the zero dispersion wavelength is near 1310 nm as shown by the dashed in FIG. 1, while wavelength division multiplexing optical transmission system usually works near 1550 nm of wave band C. More specifically, the wavelength range of wave band C is from 1529.16 nm to 1560.61 nm and the dispersion coefficient near the transmission wavelength of 1550 nm is 17 ps/nm/km. To reach 640 km transmission required by OSNR, the dispersion will accumulate to 11000 ps/nm much larger than the dispersion tolerance value of 10 Gb/s laser light source (e.g. the dispersion tolerance value of 80 km electric absorption laser is 1600 ps/nm at present). Therefore, the high speed wavelength division optical transmission system can satisfy the actual transmission requirements only after dispersion compensation. The real curve in FIG. 1 is a dispersion coefficient curve of fiber after dispersion compensation. Obviously, dispersion compensation fiber can periodically make the accumulating dispersion of fiber link to reach minimum only near a certain central wavelength of wave band C. However, the dispersion coefficient of other wavelengths on two sides of the central wavelength, especially edge wavelengths (e.g. 1529.16 nm and 1560.61 nm) is large and after transmission of multi-section, residual dispersion will increase greatly.
At present, the high speed wavelength multiplexing optical transmission system can usually perform dispersion compensation in multiplexing section or single channel. When performing dispersion compensation in multiplexing section, as shown in FIG. 2, DCM (dispersion compensating module) is periodically added in the transmission link, comprising:
Forward dispersion compensation mode, which uses forward dispersion compensation module (fiber dispersion compensation module is set before post optical amplifier) to perform dispersion pre-compensation;
Circuit dispersion compensation mode, which uses circuit dispersion compensation module (fiber dispersion compensation module is set between circuit optical amplifiers) to perform line dispersion compensation;
Backward dispersion compensation mode, which uses backward dispersion compensation module (fiber dispersion compensation module is set after of optical pre-amplifier) to perform dispersion post-compensation.
For wavelength division multiplexing optical transmission system of middle or short distance, the above methods can solve the problem of fiber dispersion to make the residual dispersion of transmission system within the range of dispersion tolerance permitted by the receiver.
However, as the relative dispersion slopes between common fiber and dispersion compensation module are not always the same, in the condition of large amount of channels and long transmission distance, even if residual dispersion of a channel of a certain wavelength is completely optimized to have the lowest bit error rate, residual dispersion of other channels (especially edge wavelength channels) may also be large, which can hardly avoid bit errors after transmission. FIG. 3 is a schematic view showing residual dispersion of wavelength division multiplexing optical transmission system, wherein D+ and D− are the ranges of dispersion tolerance permitted by receiver and D+max and D−max respectively represent the accumulating maximums of positive residual dispersion and negative residual dispersion. As shown in FIG. 3 dispersion compensation fiber can only compensate the accumulating dispersion of a certain central wavelength to minimum, while large accumulating residual dispersion still exits in other central wavelengths. Therefore, even if tunable dispersion compensator is used in the multiplexing section of wavelength division system, it is difficult to exactly compensate dispersion of all channels to optimum, so that over-compensation and lack-compensation of dispersion of each channel can hardly be avoided. The accumulation of residual dispersion is very harmful to optical transmission system of super long distance and further even break down the optical transmission system.
When performing dispersion compensation on each wavelength channel respectively with dispersion compensation modules with different compensation amounts in the wavelength channel layer, the dispersion compensation modules are usually integrated at receiving terminal of wavelength division multiplexing optical transmission system, which has compensational function to the effect of residual dispersion of the system. FIG. 4 shows a method of residual dispersion compensation of high speed wavelength multiplexing optical transmission system using EDC (electrical dispersion compensation) device.
However, the problem of the method above is: the dispersion compensation amount of electrical dispersion compensation device is fixed and very limited, which can reduce the effect brought by system residual dispersion in some degree, but can not essentially solve the problem of residual dispersion existing in the system and dynamically regulate dispersion compensation amount of each wavelength channels of wavelength division multiplexing system. As its dispersion compensation amount is not adjustable, this method lacks flexibility in the application of actual wavelength division multiplexing optical transmission system.